Flux function added solder anisotropic conductive films (ACFs) for high power and fine pitch assemblies

In this study, flux function added solder ACF was proposed in order to be used not only with the ultrasonic bonding method which can break solder using ultrasonic vibration oxides but also with conventional thermo-compression bonding method. Since the solder oxides cannot be broken only with conventional thermo-compression method, we had to remove the oxide chemically. The test vehicles were consist of polyimide based flexible substrates and FR-4 organic rigid boards which have 300 μm pitch Cu patterns with electroless nickel immersion gold (ENIG) surface finish. Solder ACFs 50 μm thick were epoxy based adhesive films containing flux functional additive. The films contain 5-15 μm diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particles which are used as conductive particles and 5 μm diameter Au coated Ni particles which are used as spacers. According to the experimental results, the flux functional additive did not change the curing reaction of the solder ACF. Moreover, the addition of flux functional additive caused significant improvement of electrical properties of solder ACF joints such as power handling capability and reliability. Therefore, using a flux function added solder ACFs can be used for various FOB and FOF assemblies and can provide an alternative interconnection for high power and fine pitch assemblies.